Photoresist strip and residue removal are critical in integrated circuit (IC) fabrication. During polysilicon etching in a typical integrated circuit fabrication process, undesirable etch residues and/or polymers such as hydrocarbon, oxide and/or silicon residues (e.g. SixBrxClxOx) may be formed and left on the surfaces and sidewalls of the resulting polysilicon structure. Such undesirable residues along with the remaining post-etch photoresist must be removed to prevent oxidation quality issues, adhesion problems, and/or diffusion contamination in subsequent integrated circuit fabrication steps.
Post-etch photoresist and polymer residue removal over polysilicon presents numerous challenges. Traditional wet chemical cleaning methods which use cleaning solutions such as dilute HF (50:1–1000:1 HF:H2O), “hot Piranha” (90% H2SO4/10% H2O2), and other corrosive solvents are often ineffective at removing all undesirable polymer residues. Depending on the application, these wet cleaning methods can over etch exposed layers or under etch (clean or strip) any remaining photoresist and polymer residue. This can cause device reliability problems or lead to nonfunctional circuits. These traditional wet clean methods also have the disadvantage of requiring handling and exposure to corrosive, oxidizing, and strongly acidic media, thus requiring extensive abatement and environmental controls.
Plasmas based on fluorine and oxygen have been used for removing post etch photoresist and polymer residues, particularly organics. Fluorine radicals and ions in such plasmas react with the organic molecules to volatilize, rendering them more water soluble and weaking their bonding structure toward plasma removal via physical etch. Molecular oxygen can aggressively remove polymer layers.
Although dry plasma cleaning processes offer an alternative approach to wet chemistry cleaning techniques, oftentimes conventional dry plasma chemistries provide insufficient cleaning, require lengthy process times, and importantly exhibit poor selectivity to exposed layers. This selectivity problem is exemplified in the over stripping of gate oxide during removal of post-etch photoresist and polymer residues. Cleaning effectiveness and selectivity are often sacrificed in the name of throughput. For example, high temperature plasma methods often remove unwanted residues and upper layers on a device quickly, but are often hard to control and thus unacceptable amounts of oxide or other underlying layers are lost. This issue is becoming increasingly important with integrated circuit device structures becoming smaller and smaller.
Cleaning wafers with various types of plasmas and plasma power sources have been described. Recently Pavel (“Combining microwave downstream and RF plasma technology for etch and clean applications”, Proceedings of the International Symposium, Electrochemical Society Proceedings Vol. 99–30) described the use of a two-step process for etch and clean applications. She reported that treatment of a wafer with a RF generated oxygen-based plasma followed by application of a microwave (MW) generated plasma at high temperature was effective at cleaning some post etch and post high dose implant strip layer with enhanced etch rates. This method has the disadvantage of oxidizing polymer residues due to the exposure to high temperatures (200–300° C.), making them harder to remove in subsequent steps. Many low-k materials are also sensitive to high temperature process steps and so cannot be used with such high temperature plasma processing.
What is needed therefore are improved methods of cleaning or treating the surface of semiconductor wafers during integrated circuit fabrication. Preferably low temperature methods that not only effectively remove unwanted layers from integrated circuit devices, but also show selectivity by not over stripping desired layers from the devices.